1. Field
The present disclosure relates to the field of molds for molding and vulcanizing a tire tread. More specifically, the disclosure relates to the molds used for molding in the tread blocks which are partially covered by a layer of a covering material.
2. Description of Related Art
It is known practice to design tires the tread of which comprises various rubber compounds. Document WO 03089257 discloses such treads. More specifically, document WO 03089257 discloses a tread comprising tread blocks the walls of which are covered with a layer of covering material. The material of which this cover layer is made differs from the rubber compound of which the tread is made. This material notably has much better wet grip than the wet grip of the rubber compound. This allows a very significant improvement in cornering performance on wet ground.
One way of manufacturing this tread is notably disclosed in document WO 2006069912. According to this method of manufacture, in a first step, provision is made for the material intended to constitute a layer of covering material to be injected in the form of one or more inserts into the green tire using an injection nozzle. The insert or inserts is or are then shaped, in a second step, by ribs of a vulcanizing mold so that they cover all or part of the walls of the grooves molded by these ribs.
This method of manufacture has its limits, particularly as far as obtaining precision moldings is concerned. Specifically, during the shaping thereof, the insert experiences a significant shear force from the rib in order to convert this insert into a layer of lesser thickness. This shear force may cause cracking within the insert, making it more difficult to control the movements of the material of which this insert is made. The shape and the thickness of the layer of covering material thus formed may therefore be somewhat haphazard. The advantages afforded by the additional layer to the performance of the tire are then reduced.
In addition, in this method of manufacture, it is necessary to make the inserts align with the ribs. This makes manufacture of the tread more complicated.
Document FR11/61796 discloses a method of manufacturing a tire which involves cutting a layer of cover material laid beforehand on a green tire and placing part of this cover layer into a cut molded by a blade of a molding element. In that method, the layer of covering material is cut and forced into the depth of the green tire in one and the same movement of the molding element with respect to the green tire.
In order to improve the grip of a winter tire, also known as a snow tire on snow-covered ground it is known practice to use within the tread a rubber material that has a low elastic modulus. The overall rigidity of the tread blocks that make up this tread is therefore reduced, and this may also reduce their resistance to wear.
In order to improve the rigidity of the blocks while at the same time maintaining good grip on snowy ground, it is known practice to form a tread with inclined blocks making an angle greater than 0° with an axial direction of the tire. In this way, each block offers a greater apparent beam section with respect to the loadings applied to this block.
Again with a view to improving the rigidity of the blocks, it is known practice to provide the lateral faces of the blocks with a layer of a covering material that has an elastic modulus higher than the elastic modulus of the rubber material.
However, manufacturing a snow tire comprising such inclined blocks using the method of manufacture of document FR11/61796 presents a certain number of difficulties. In particular, it has been observed that the shape of the cut-outs in the cover layer do not match well with the shape of the lateral walls of the inclined blocks intended to accept this cover layer, and this leads to problems regarding the way in which the tire behaves.
There is therefore a need to improve the incorporation of a layer of a covering material on the walls of inclined tread blocks of a tire tread.